A jitter test is one of many very important tests with regard to a serial communication device or serial I/O device. For example, the recommendation of International Telecommunication Union, Bellcore or the like specifies the measurement of jitter tolerance, jitter generation, and jitter transfer function. Particularly, the jitter tolerance test is important because this test makes it possible to evaluate the behavior limit of a device to the jitter added to a transmission medium. The measurement of jitter tolerance is to measure the magnitude of the jitter applied to an input signal, by which a bit error begins to occur in a device, with the magnitude of the applied jitter being changed.
FIG. 1 shows a conventional measurement of jitter tolerance. In the conventional measurement of jitter tolerance, random jitter is applied to an input signal shown in FIG. 1A by overlapping white noise shown in FIG. 1B and the input signal. FIG. 1C shows the input signal to which the random jitter applied. Then, the input signal, to which the random jitter applied, is supplied to an electronic device so that whether or not a bit error occurs in the electronic device is measured.
FIG. 2 shows a configuration of a conventional apparatus 200 for applying jitter to an input signal. The input signal generated by a pattern generator 202 is applied with sinusoidal jitter by a sinusoidal jitter source 206, and further applied with deterministic jitter and random jitter by a deterministic jitter source 208 and a random jitter source. The jitter amount to be applied to the input signal is adjusted according to the magnitudes of the random jitter and the sinusoidal jitter. Then, the input signal is amplified by a limiting amplifier 214, and is outputted after the amplitude components of the input signal, which are more than or equal to a predetermined value or less than or equal to another predetermined value, are clipped.
FIG. 3 shows behavior of the limiting amplifier 214. The limiting amplifier 214 is given the input signal shown in FIG. 3A. The input signal has amplitude variation components because it is applied with the random jitter.
Although the limiting amplifier 214 reduces the amplitude variation components by eliminating the amplitude components more than or equal to a first threshold and the amplitude components less than or equal to a second threshold among the amplitude components of the input signal as shown in FIG. 3B, the amplitude variation components that are within a range less than the first threshold and more than the second threshold cannot be eliminated. In order to measure the jitter tolerance of an electronic device, it is necessary to supply an input signal without any amplitude variation component like the input signal shown in FIG. 3C to the electronic device and detect a bit error caused by only the jitter components in the amplitude direction. However, the bit error caused by the amplitude variation components is bound to be detected in the conventional apparatus 200 because the amplitude variation components remain in the input signal as shown in FIG. 3B. Accordingly, the jitter tolerance of the electronic device is evaluated too small. In addition, since the conventional apparatus 200 for applying jitter has the three jitter sources, i.e., the sinusoidal jitter source 206, the deterministic jitter source 208, and the random jitter source 212, the manufacture cost is large.
Therefore, it is an object of the present invention to provide a testing apparatus and a testing method, which is capable of overcoming the above drawbacks accompanying the conventional art. The above and other objects can be achieved by combinations described in the independent claims. The dependent claims define further advantageous and exemplary combinations of the present invention.